The present invention relates generally to video recording and reproducing apparatus, and more specifically to a jitter-immune circuit for expanding a time-compressed, line-sequentially multiplexed video signal.
Current video recording techniques involve modulating the frequency of the luminance component of a composite color television signal, converting the frequency of the chrominance component to a lower frequency in the video spectrum and multiplexing the luminance and chrominance components in a frequency-division multiplexed format. However, the desire to achieve high quality video reproduction has brought about a time compression and expansion technique in which the luminance and color-difference components are time-compressed, line-sequentially multiplexed and modulated in frequency upon a carrier having a higher frequency than the carrier employed in the frequency-division multiplex recording. Since the luminance and color-difference signals do not coexist at the same time, the time compression technique has advantages of avoiding undesirable interference between them and avoiding undesirable Moire fringe pattern inherent in the frequency-division multiplex recording. Furthermore, the use of the higher frequency carrier eliminates the azimuth loss which is likely to cause crosstalk between adjacent tracks. A further advantage of the time compression technique is that it results in a spectral distribution of energies having a higher concentration in the lower frequency band of the spectrum and a lower energy concentration in the higher frequency band. This profile of spectral disctribution lends itself to frequency modulation, resulting in a higher modulation index and a significant improvement in signal-to-noise ratio.
The time-compression technique requires read-write memories for writing and reading signals at sequentially varying rates for time compression and expansion under control of address counters. Because of the line-sequential signal format, the address counters need to be initialized at the beginning of each horizontal scan to align the timing of write operation with the timing of read operation. However, due to mechanical tolerances inherent in recording apparatus, reproduced signals are severely affected by jitter and noise, causing the horizontal sync to deviate from the reference time and hence the occurrence of a misregistration between the recovered luminance and color components. One approach would be to employ a time base corrector. However, the cost is prohibitive for extensive household applications.